This invention relates in general to molded-case circuit breakers and, more particularly, to an undervoltage release device assembly which is installed in such a circuit breaker.
More specifically, the invention concerns a circuit breaker known as the molded-case type, namely having a housing, or case, molded from insulating material. For the purpose of describing such a circuit breaker with an undervoltage trip mechanism embodied therein, reference is made to U.S. Pat. Nos. 2,697,148 and 4,620,171. These patents are hereby incorporated by reference.
Designing such a circuit breaker to increase its capability and generalize its application has been a challenge because of the relatively small space available within the housing of a molded-case circuit breaker, and the limits to which the overall dimensions of a circuit breaker housing can be increased without rendering the circuit breaker impractical for use in the kind of environment typical for molded-case breakers.
One main object of the present invention is to provide in a molded-case type circuit breaker an improved undervoltage release device which is reliable, requires relatively little space and has a longer useful life.
An electrical circuit breaker incorporates an operating mechanism for closing or opening the contacts in response to circuit breaker handle movement caused by external means. The circuit breaker mechanism also opens the contacts by release of a trip member in response to an overcurrent condition. The circuit breaker is provided with an undervoltage release device overriding any other operating condition in order to actuate a trip member upon the occurrence of an undervoltage condition. As generally known, the undervoltage release device, typically, comprises: an electromagnetic coil surrounding a plunger disposed axially for movement between a reset position and an actuated position, with a spring biasing the plunger toward the actuated position whenever an undervoltage condition takes away the coil's magnetic holding force and releases the bias spring. The plunger, when actuated, will engage a trip member of the circuit breaker, causing tripping of the circuit breaker. The plunger is inductively coupled with the coil inducing a magnetic force sufficient to hold the plunger in its reset position, i.e. away from the trip member, provided the coil is supplied a voltage of predetermined value. Should the voltage decrease under the limit, the force exercised upon the plunger will decrease and the biasing spring will take over, thereby moving the plunger to the actuated position. The undervoltage release device is required to embody only a few major components compactly arranged. Nevertheless, it must be capable of performing several functions. The main function is to reliably trip the circuit breaker open when an undervoltage condition occurs. In this regard, it utilizes a lever which is actuated by the handle of the circuit breaker in order to reset the plunger into its home position, against the biasing spring, as soon as the voltage applied to the coil has been restored. When the plunger is in its actuated position, i.e. under the biasing spring, the plunger is projecting outside of its core and of the coil associated cylindrical member. Resetting is effectuated by pushing with the lever the plunger back into its core and against the biasing spring. Resetting of the plunger through the lever is commanded by the handle of the circuit breaker, the lever being operatively disposed between an actuating member of the handle and an open end of the actuating plunger. The plunger open end is also used to actuate the tripping mechanism of the circuit breaker in order to translate the plunger action of the undervoltage release device into movement of the trip member of the circuit breaker.
The resetting lever of the undervoltage release device consists of thermoplastic material molded with two ends, one to receive the impact of the plunger when actuated by the undervoltage release device, the other to be actuated upon by the handle of the circuit breaker for resetting the undervoltage release device. Testing of the circuit breaker and of the undervoltage release device will put to the test the quality and effectiveness of the reset lever. Testing of the circuit breaker includes a thermal test in which the circuit breaker is soaked in an oven at an elevated temperature for an extended time period in the tripped position, then, cooled to ambient temperature with the breaker in the OFF position. Moreover, the resetting lever should be capable of going through an endurance test involving over 14,000 resetting operations. Therefore, it is important to minimize stress and creep in the resetting lever.